Other mistake: When the pirates are thrown off the boat by Peter, after being kissed by Wendy, the splash is not heard and they cannot be seen in the water when Peter flies up into the air.
Other mistake: At the very end of the movie, Peter says "to hear stories about me" and we see Wendy and her eyes are moving as if watching him fly away, but when it cuts back to Peter he then starts to fly away.
Other mistake: When John and Michael are explaining Hook to Aunt Millicent, Michael mouths some of John's lines. (00:03:45)
Other mistake: When asking Wendy to be their mother, one of the twins asks, "Do you tell stories?" and Curly visibly mouths the word, "Stories," along with the twin, then goes on to say his own line, "Then you're perfect." (00:37:40)
Other mistake: When the Lost Boys spot the large white bird they argue about who gets the telescope. In the wide shot one of the twins slips and falls on the ground and looks directly at the camera with a puzzled look on his face. In the next close-up Tootles and especially Nibs start smirking, Nibs has to look down and away as Slightly says his lines and the other twin starts to look at the camera when the shot changes. (00:32:25)
Other mistake: In the scene where Wendy admits to being Red-Handed Jill, Tootles says "Mother and Father are fighting again" and Curly is mouthing his lines.
Other mistake: After the lost boys put Wendy into the doorway of the treehouse for the first time, right after the one closing the door says, "So looking forward to being your son!": the lost boy right behind him turns, and pulls back his bowstring as if aiming, but has neglected to knock an arrow on the string. Then, as if that wasn't bad enough, he turns toward the camera and makes a funny face. (00:38:15)
Answer: It's not that you never age, it's that you never grow up, i.e. pass into puberty and become an adult.
Myridon